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+=====================================================
+Freescale i.MX8 DDR Performance Monitoring Unit (PMU)
+=====================================================
+
+There are no performance counters inside the DRAM controller, so performance
+signals are brought out to the edge of the controller where a set of 4 x 32 bit
+counters is implemented. This is controlled by the CSV modes programmed in counter
+control register which causes a large number of PERF signals to be generated.
+
+Selection of the value for each counter is done via the config registers. There
+is one register for each counter. Counter 0 is special in that it always counts
+“time” and when expired causes a lock on itself and the other counters and an
+interrupt is raised. If any other counter overflows, it continues counting, and
+no interrupt is raised.
+
+The "format" directory describes format of the config (event ID) and config1/2
+(AXI filter setting) fields of the perf_event_attr structure, see /sys/bus/event_source/
+devices/imx8_ddr0/format/. The "events" directory describes the events types
+hardware supported that can be used with perf tool, see /sys/bus/event_source/
+devices/imx8_ddr0/events/. The "caps" directory describes filter features implemented
+in DDR PMU, see /sys/bus/events_source/devices/imx8_ddr0/caps/.
+
+    .. code-block:: bash
+
+        perf stat -a -e imx8_ddr0/cycles/ cmd
+        perf stat -a -e imx8_ddr0/read/,imx8_ddr0/write/ cmd
+
+AXI filtering is only used by CSV modes 0x41 (axid-read) and 0x42 (axid-write)
+to count reading or writing matches filter setting. Filter setting is various
+from different DRAM controller implementations, which is distinguished by quirks
+in the driver. You also can dump info from userspace, "caps" directory show the
+type of AXI filter (filter, enhanced_filter and super_filter). Value 0 for
+un-supported, and value 1 for supported.
+
+* With DDR_CAP_AXI_ID_FILTER quirk(filter: 1, enhanced_filter: 0, super_filter: 0).
+  Filter is defined with two configuration parts:
+  --AXI_ID defines AxID matching value.
+  --AXI_MASKING defines which bits of AxID are meaningful for the matching.
+
+      - 0: corresponding bit is masked.
+      - 1: corresponding bit is not masked, i.e. used to do the matching.
+
+  AXI_ID and AXI_MASKING are mapped on DPCR1 register in performance counter.
+  When non-masked bits are matching corresponding AXI_ID bits then counter is
+  incremented. Perf counter is incremented if::
+
+        AxID && AXI_MASKING == AXI_ID && AXI_MASKING
+
+  This filter doesn't support filter different AXI ID for axid-read and axid-write
+  event at the same time as this filter is shared between counters.
+
+  .. code-block:: bash
+
+      perf stat -a -e imx8_ddr0/axid-read,axi_mask=0xMMMM,axi_id=0xDDDD/ cmd
+      perf stat -a -e imx8_ddr0/axid-write,axi_mask=0xMMMM,axi_id=0xDDDD/ cmd
+
+  .. note::
+
+      axi_mask is inverted in userspace(i.e. set bits are bits to mask), and
+      it will be reverted in driver automatically. so that the user can just specify
+      axi_id to monitor a specific id, rather than having to specify axi_mask.
+
+  .. code-block:: bash
+
+        perf stat -a -e imx8_ddr0/axid-read,axi_id=0x12/ cmd, which will monitor ARID=0x12
+
+* With DDR_CAP_AXI_ID_FILTER_ENHANCED quirk(filter: 1, enhanced_filter: 1, super_filter: 0).
+  This is an extension to the DDR_CAP_AXI_ID_FILTER quirk which permits
+  counting the number of bytes (as opposed to the number of bursts) from DDR
+  read and write transactions concurrently with another set of data counters.
+
+* With DDR_CAP_AXI_ID_PORT_CHANNEL_FILTER quirk(filter: 0, enhanced_filter: 0, super_filter: 1).
+  There is a limitation in previous AXI filter, it cannot filter different IDs
+  at the same time as the filter is shared between counters. This quirk is the
+  extension of AXI ID filter. One improvement is that counter 1-3 has their own
+  filter, means that it supports concurrently filter various IDs. Another
+  improvement is that counter 1-3 supports AXI PORT and CHANNEL selection. Support
+  selecting address channel or data channel.
+
+  Filter is defined with 2 configuration registers per counter 1-3.
+  --Counter N MASK COMP register - including AXI_ID and AXI_MASKING.
+  --Counter N MUX CNTL register - including AXI CHANNEL and AXI PORT.
+
+      - 0: address channel
+      - 1: data channel
+
+  PMU in DDR subsystem, only one single port0 exists, so axi_port is reserved
+  which should be 0.
+
+  .. code-block:: bash
+
+      perf stat -a -e imx8_ddr0/axid-read,axi_mask=0xMMMM,axi_id=0xDDDD,axi_channel=0xH/ cmd
+      perf stat -a -e imx8_ddr0/axid-write,axi_mask=0xMMMM,axi_id=0xDDDD,axi_channel=0xH/ cmd
+
+  .. note::
+
+      axi_channel is inverted in userspace, and it will be reverted in driver
+      automatically. So that users do not need specify axi_channel if want to
+      monitor data channel from DDR transactions, since data channel is more
+      meaningful.